Biden on Thursday announced his slate for new leadership at the Democratic National Committee, including former South Carolina U.S. Senate candidate Jaime Harrison to be the next chair of the DNC.

Harrison, who lost the 2020 race to GOP Sen. Lindsey Graham last year, despite breaking numerous fundraising records, will officially become DNC chair after the full committee votes on Jan. 21, the day after Biden's inauguration.

Graham congratulated Harrison on the news in a tweet earlier Thursday.

In addition to Harrison, Biden also announced the names of four DNC vice chairs, all of whom were prominent backers of his presidential campaign, including Michigan Gov. Gretchen Whitmer, who served as a campaign co-chair.

Whitmer is joined by Atlanta Mayor Keisha Lance Bottoms, Illinois Sen. Tammy Duckworth and Texas Rep. Filemon Vela as new DNC vice chairs.

House Speaker Nancy Pelosi called Harrison an "inspiring, energizing and influential" in a press release.

"As former Chair of the South Carolina Democratic Party and senatorial candidate, Jaime brings an outstanding track record of success -- from galvanizing the grassroots, mobilizing millions and shattering fundraising records -- that will be critical during this pivotal moment for our Party and Country," she added.

-ABC News' John Verhovek and Beatrice Peterson

During an unscheduled stop on Capitol Hill, Vice President Mike Pence greeted National Guardsmen and telling them that he had just come from a briefing where he heard about their efforts.

He thanked the personnel for "stepping forward for your country" to make the historic transfer of power possible.

"It's been my great honor to serve as your vice president and I want to thank you for your service," Pence said.

The vice president then spoke to individual guardsmen asking them where they are from and how long they served.

About 20,000 National Guard members are expected in Washington this week to protect the city ahead of Biden's inauguration.

"Have a safe inauguration," Pence said.

-ABC News' Elizabeth Thomas and John Parkinson

Congressional Democratic leaders commended Biden's announcement outlining his COVID-19 emergency relief proposal.

"The emergency relief framework announced by the incoming Biden-Harris Administration tonight is the right approach. It shows that Democrats will finally have a partner at the White House that understands the need to take swift action to address the needs of struggling communities," House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer said in a statement.

"We will get right to work to turn President-elect Biden's vision into legislation that will pass both chambers and be signed into law," they added.

"The robust package announced by President-elect Biden and Vice President-elect Harris shows we now have the necessary leadership in the Executive Branch to turn this around and we are ready to get to work to deliver aid without delay," Pelosi and Schumer said.

-ABC News' Mariam Khan

Biden will propose a $1.9 trillion, “American Rescue Plan" to address the ongoing COVID-19 vaccination effort and immediate economic need to Americans in evening remarks, transition officials disclosed on a call with reporters Thursday.

According to officials on the call, the rescue policy is only one part of Biden’s overall policy, with a “recovery” phase expected to be introduced in February. They said Biden’s rescue policy will include three components: Tackling vaccine distribution, ramping up testing and getting kids back in school within the first 100 days of his administration; delivering $1,400 per-person direct stimulus checks and expanding unemployment benefits, including $400 weekly benefit through September on top of state benefits; and delivering aid to states and communities to support small businesses, addressing housing insecurity by expanding the eviction and foreclosure moratorium through Sept. 2021.

More than $1 trillion of the $1.9 trillion price tag will be used for direct stimulus, officials said, while $400 billion will go toward COVID-19-related projects, including the nationwide vaccination program, and $440 billion will go toward relief for communities and businesses.

Biden will also call for Congress to provide $350 billion for state local and territorial governments, an effort to keep essential workers on the job.

If enacted, it would be one of the largest and most expensive economic stimulus packages in U.S. history.

-ABC News Molly Nagle, John Verhovek, Beatrice Peterson and Averi Harper

Addressing the coronavirus pandemic, confronting the climate crisis and building public trust in science and technology will be among the Biden administration's key areas of focus, the president-elect said during a press briefing introducing four key members of his science team.

Building back the economy "to ensure prosperity is fully shared all across America" and ensuring the U.S. "leads the world in technologies and industries that the future" will also be a focus, Biden said Saturday during an address in Wilmington, Delaware.

Helping lead those initiatives will be Eric Lander, the presidential science advisor-designate, a position that for the first time will be a cabinet rank, Biden noted.

"I've always said that the Biden-Harris administration will lead with science and truth," Biden said. "We believe in both. This is how we're going to, God willing, overcome the pandemic and build our country back better than it was before. That's why for the first time in history, I'm going to be elevating the presidential science advisor to a Cabinet rank, because we think it's that important."

Frances Arnold and Maria Zuber will be at the helm of the first all-women team to lead the President’s Council of Advisors on Science and Technology.

Alondra Nelson will also be deputy director for science and society for the Office of Science and Technology Policy.

Dr. Francis Collins, who was not present at the event, will also continue in his role as director of the National Institutes of Health.Biden thanked him for "being willing to stay on," saying that it wasn't his original plan.

"I've known Dr. Collins for many years. I worked with him closely," Biden said, accling him "brilliant, a pioneer, a true leader."

"This is the most exciting announcement that I have gotten to make in the entire Cabinet," Biden said of his picks. "These are among the brightest, most dedicated people, not only in the country but the world."